In the promotion of many paper or textile products such as personal care products, the perceived texture and softness of the product by the consumer is important for its general acceptance and use. To obtain this desired texture and softness, a condition that is usually loosely defined and difficult to describe, manufacturers expend considerable time and effort adjusting their manufacturing process to produce materials with a fiber content, size, and dispersion that ultimately "feels soft" in the final product. Often, the determination of texture and softness is accomplished by a team of human evaluators who touch and manipulate product samples to evaluate the quality and acceptability of the material. While the judgment is mostly qualitative, this typical approach to quality assurance has generally worked well in the past.
Unfortunately, the use of human evaluators has a number of limitations. The decision of whether or not a material has an acceptable texture and softness is largely qualitative, and the number of process variables involved in achieving this softness is large. Often it is difficult, without considerable analysis, to precisely locate operations within the manufacturing process that may require adjustment. Furthermore, because human beings are subject to sickness, emotional stress, and dermatological ailments, among others, the judgment of texture or softness made by the evaluation team is not always accurate and repeatable. This fluctuation in judgment can be detrimental to the manufacturer in two ways: (1) If good material is judged as unacceptable, considerable time and resources are needlessly wasted in trying to correct the situation. (2) More importantly, if poor material is judged as acceptable, the product may eventually be rejected by the consumer, adversely affecting the manufacturer's reputation.
The need for developing an integrated softness parameter much like the numerical scoring techniques used to rank standard paper tissue or textile product softness is accordingly apparent. Such may be accomplished by monitoring the dynamic characteristics of two similar samples when they are gently rubbed together, and advances in sensing technology and signal processing offer solutions to the problem.
The prior art of softness sensing, particularly of textile materials, contains several examples of materials that are rubbed to generate an acoustic emission, which in turn is evaluated to determine the characteristics of the material being tested. One approach is described by Thorsen in "Apparatus for Determining Textile Characteristics " (U.S. Pat. No. 2,752,781), where two similar materials under test are rubbed against each other and the acoustic output is monitored with a vibrating diaphragm and contact microphone. A similar approach is described by Veneklasen, et al, in "Acoustic Testing Instrument" (U.S. Pat. No. 2,922,303) but relies on fluid coupling to transmit the acoustic signals. In Taylor's patent entitled "Method and Apparatus for Softness Testing" (U.S. Pat. No. 3,683,681), a third approach, specifically designed for measuring softness of paper fabric materials, is disclosed.
In the Thorsen patent, vibration resulting from two similar materials drawn across each other is transmitted through a cardboard diaphragm to a contact microphone. Several means of vibration isolation are employed, and the entire measuring apparatus is surrounded by sound absorbing material to reduce the effects of ambient noise.
The texture softness sensing approach disclosed in this application also relies on mutual motion between two materials under test, but the sensing element is a flexible piezoelectric polymer that is in intimate contact with the materials. By using this technique, the sensitivity to acoustic vibration in the tested materials is maximized, and minimal means of vibration isolation are required. In addition, while the sensor is capable of detecting some ambient noise, the configuration is non-optimum for this mode of detection. The net result is that ambient conditions have minimal effect on the measurement technique, and little if any means of acoustic shielding are needed.
The sensing approach was principally designed to measure the surface characteristics of tissue paper products, but it may also be applied to textile materials with equal validity.
Finally, the disclosed technique is designed to measure surface characteristics of pliable materials, in particular surface softness. But, the principal material parameter measured by the approach described by Taylor is generally referred to as bulk softness. Bulk softness is a characteristic of paper and textile products that relates to the perceived stiffness, density, and general handling of the material. Furthermore, the methods of measurement are varied. Taylor's apparatus consists of multiple rolls that massage and distort the bulk features of the materials under test so that acoustic emissions are generated and detected by a remotely place microphone. In the texture sensing approach described here, surface softness is measured using an intimately contacting acoustic sensor.